The invention relates to a grease to be filled in a fan bearing. The term "fan" used in this specification refers to a device for performing a forced cooling relative to cooling water in an engine of an automobile.
It is known that performance and life of a rolling bearing, which is filled with a quantity of grease and sealed for example by a rubber seal, depend on the properties of a grease to be used. A ball bearing for supporting a fan, which performs a forced cooling relative to cooling water in an engine of an automobile, is also filled with a quantity of grease. Thus, it is believed that performance and service life of such a ball bearing are also influenced by the properties of a grease to be used.
Recently, miniaturization and lightening for auxiliary parts of an automobile are facilitated in order to meet a demand for lightening of an automobile. At the same time, such auxiliary parts are also requested to have a high output and a high efficiency. Thus, the reduction in output power of the auxiliary part, due to the miniaturization, is usually compensated for by making its rotational speed higher, it is also noted that the degree of sealing for an engine room is increased, in order to meet the requirement for quieting, so that the temperature within the engine room tends to be increased. Thus, the parts within the engine room are also required to have a durability at higher temperatures.
A fan 1 for an automobile 1 includes, as shown in FIGS. 3(a) and 3(b), an annular casing (rotating member) 6 and a plurality of blades 2 attached to the periphery of the annular casing and extending radially therefrom. The casing 6 is fitted over an outer ring 3a of a ball bearing 3. The fan is rotatably mounted on a main shaft 4 to be driven by an engine. For example, the main shaft 4 is bolted to a flange portion of a pump shaft. A clutch mechanism 5 is disposed between the main shaft 4 and the fan 1. The clutch mechanism 6 is of a temperature sensitive type, so that it is capable of being engaged and disengaged in accordance with variation in the outside air temperature. A torque is transmitted from the main shaft 4 through the clutch mechanism 5 to the fan 1. One example for the construction of the clutch mechanism 5 will be explained below.
A drive disk 7 is securely fitted over the forward end of the main shaft 4 for unitary rotation therewith. A disk-like plate 8 is mounted on the drive disk 7 at its forward end. The plate 8, together with the casing 6, defines a first chamber 9 serving as a torque transmission chamber. A second chamber 11, serving as a reservoir for oil (viscous fluid for the transmission of torque, such as silicone oil), is defined between the plate 8 and a cover 10. A leaf spring 13 is disposed within the second chamber 11. The leaf spring 13 has one end attached to a valve 12 and the other end secured to the plate 8. A bimetal 14 is secured to the cover 10. The bimetal 14 is fitted with a piston 15 which is in alignment with the axis of the main shaft 4.
With the above-mentioned construction, the bimetal 14 is not significantly curved as shown in FIG. 3(a), when the outside air temperature is low. Thus, the piston 15 is forced to urge the leaf spring in the right-hand direction in the drawing. At this time, the valve 12 attached to the leaf spring extends through an aperture 16 formed in the peripheral portion of the plate 8 and is urged against the forward surface of the drive disk 7 within the first chamber 9. The aperture 16 includes, as shown in FIG. 4, an insertion hole 16a and a communication hole 16 in continuation with the insertion hole 16a. The valve 12 is fitted within the insertion hole 16a. A relative velocity is always present between the drive disk 7 and the casing 6, i. e., valve 12. Accordingly, the oil (shown by dots) between the drive disk 7 and the plate 8 is driven, by means of a scraping action of the valve 12 serving as a weir, to be flown through the communication hole 16b into the second chamber 11. Under this condition, substantially the whole quantity of the oil in the first chamber 9 is flown into the second chamber 11, thus only a small quantity of oil remaining in the first chamber 9. Accordingly, the amount of torque transmission is reduced and the rotational speed of the fan 1 is decreased. It is noted that the arrow mark in FIG. 4 indicates the direction in which the drive disk 7 is rotated.
The bimetal 14 is curved as shown in FIG. 3(b), when the outside air temperature is high. Thus, the valve 12 is moved in the left-hand direction of the drawing, so that the piston 15 becomes escaped from the aperture 16 by reason of the resiliency of the leaf spring 13. Accordingly, the above-mentioned scraping action becomes ineffective. By this, the surfaces of the oils within the first and second chambers 9, 11 become to be at the same level, by reason of the communication through the aperture 16. At this time, the effective transmission surface of the first chamber 9 is filled with oil, so that a predetermined torque is transmitted and the rotational speed of the fan 1 is increased.
In the above-mentioned fan, the grease within the ball bearing 3 is required to have an extended life in terms of bearing lubrication at higher temperatures, a low leakage property, and a superior performance at low temperatures.
In order to meet the above-mentioned requirements, greases have been conventionally used which are formed by combining a urea based thickening agent with a synthetic oil as a base oil, by combining a fluoro thickening agent with a fluoro silicone oil or a fluoro oil as a base oil, or by combining a Li-soap thickening agent with a silicone oil as a base oil.
When the above-mentioned fan is operated under cold environment (in winter), an unusual sound (a hoot sound) may be generated, depending upon the specification of the fan or operation conditions. It is noted, however, that the hoot sound under cold environment is not always generated. It is also noted that the hoot sound is generated only a short time. Specifically, the hoot sound is generated upon starting an engine of an automobile and not caused thereafter. Since the hoot sound has such complex characteristics, the reason why it is caused has not yet been clarified. It is also noted that the fan used in an automobile is operated at high temperatures and high speeds, and its durability is one of the important characteristics. No effective measures for preventing generation of hoot sound, however, have been provided heretofore.
It has been studied in prior art to employ, as a countermeasure for preventing generation of hoot sound, a low viscosity grease which has a superior characteristics at lower temperatures (for example, forming an even oil film between the balls and the raceway surfaces of the inner and outer rings under cold environment, and having satisfactory flowability). The countermeasure is intended to restrict generation of the hoot sound by increasing the lubricity of the grease under cold environment. It is noted, however that such a grease has a low viscosity, so that, when a grease consisting of the combination of a synthetic oil (base oil) and an urea (thickening agent), or the combination of a silicone oil (base oil) and a Li-soap (thickening agent) is employed, it is expected that a satisfactory lubricity at higher temperatures would not be obtained and that durability would be decreased. It is also mentioned that the combination of a fluoro oil (base oil) and a fluoro thickening agent is not cost-effective.